Although it is not a cure, it can help patients who have not responded to other available HIV drugs.

HIV charities welcomed the drug's arrival, which has taken Pfizer 10 years to develop at a cost of hundreds of millions of dollars.

Novel action

A cocktail of three drugs collectively called HAART (highly active anti-retroviral therapy) has dramatically improved the life expectancy of patients with HIV, but resistance to these drugs is a problem.

Maraviroc was approved in the US in August, where it is marketed under the name of Selzentry.

It is taken in combination with other anti-retroviral drugs, but works in an entirely new way.

It blocks a microscopic doorway - the CCR5 receptor - which HIV uses to enter and infect human cells called CD4-T-cells.

All other currently available oral HIV medicines work on the HIV virus once it has entered the immune cells.

Trials show that in suitable patients - those infected with only CCR5-tropic HIV-1 - it can help reduce the levels of virus circulating in the blood stream and increase the numbers of immune CD4 cells.

Between 50 and 78% of people with HIV have this strain.

Professor Margaret Johnson, chair of the British HIV Association, said: "The introduction of maraviroc is incredibly important for the HIV community.

HIV milestones

1984 - HIV identified as cause of Aids

1987 - first antiretroviral drug becomes available in the US and UK

1992 - beginning of combination therapy for HIV

1995 - beginning of Highly Active Antiretroviral Therapy

1998 - first report of multi-drug resistant HIV

"HIV is known to mutate aggressively and some patients become resistant to the treatments available. New drugs and particularly new classes of treatment are fundamental for continued good health."

Roger Pebody of the Terrence Higgins Trust said: "Maraviroc could make a real difference to people with HIV who are resistant to other drugs."

To date, HIV has claimed more than 25 million lives. Another 40 million people are estimated to be living with HIV worldwide.